C.G.St.C. Kendall

Holocene coastal carbonates and evaporites of the southern Arabian Gulf and their ancient analogues

Abstract The Holocene sediments of the coast of the United Arab Emirates in the southeastern Arabian Gulf are frequently cited in the literature as type examples for analogous assemblages of carbonates, evaporites and siliciclastics throughout the geologic record. This paper is intended as a convenient single source for the description of sediments of this region, providing information on how to reach the classic localities and some of the analogs. The Holocene sediments of the region accumulate over an area that is 500 km long and up to 60 km wide. The sediments collecting offshore are predominantly pelecypod sands mixed with lime and argillaceous mud, with these latter fine sediments increasing as the water deepens. The pelecypod-rich sediments also collect east of Abu Dhabi Island both in the deeper tidal channels between the barrier island lagoons and in deeper portions of the protected lagoons. West of Abu Dhabi Island the shallow water margin is the site of coral reefs and coralgal sands, whereas to the east oolites accumulate on the tidal deltas of channels located between barrier islands. Grapestones accumulate to the lee of the reefs and the oolite shoals where cementation becomes more common. They are particularly common on the less protected shallow water margins of the lagoons west of Abu Dhabi Island. Pelleted lime muds accumulate in the lagoons in the lee of the barrier islands of the eastern Abu Dhabi. Lining the inner shores of the protected lagoons of Abu Dhabi and on other islands to the west are cyano-bacterial mats and mangrove swamps. Landward of these, a prograding north facing shoreline is formed by supratidal salt flats (sabkhas), in which evaporite minerals are accumulating. This paper describes the localities associated with (1) the mangrove swamps of the west side of the Al Dhabaiya peninsula; (2) the indurated cemented carbonate crusts, cyanobacterial flats and sabkha evaporites on the shore of the Khor al Bazam south of Qanatir Island; (3) the reef and oolitic sand flats on the coast just east of Jebel Dhana; and (4) the marine travertine and aragonite coats associated with the beach sediments in a small bay south of Jebel Dhana; and (5) the Sabkha Mutti between Jebel Barakah and Al Sila. Similar sedimentological associations of carbonate and evaporites to those of the Holocene of the United Arab Emirates are to be found in the Tertiary and Mesozoic sedimentary rocks of the immediate subsurface in the Arabian Gulf. Other analogs to this setting include the Paleozoic carbonates of the western USA, Europe, and Asia, Mesozoic carbonates of the Gulf of Mexico, Europe, and Middle East and Tertiary sedimentary rocks in the Middle East.

Basin Evaluation Using Burial History Calculations: an Overview

Burial history calculations available to the petroleum geologist include plots of burial history and geohistory, porosity, and conductivity as a function of depth and lithology, as well as the results obtained from algorithms to handle unconformities and backstripped crustal subsidence, calculations that predict the breakdown of kerogen to hydrocarbons in terms of time and temperature, and methods for determining heat flow from geologic models and vitrinite reflectance. Data inputs required for these calculations include depths of formation tops, ages of formation tops, lithologies of formations, depths of water at deposition of the different formations, porosities of the formations as a function of depth, bottom-hole temperatures and/or formation temperatures, and any ke ogen content and vitrinite reflectance values as functions of depth. Sources of many of these data may be well logs, well reports, geologic papers, and/or seismic sections. The paper considers how parameters can be extracted from burial-history calculations to construct maps that can be compared to the current locations of oil and gas fields, and so used to locate and rank prospective acreage. These parameters include the potential source rock maturity of a formation plotted as a function of time and depth, the thermal history of any hydrocarbons associated with the potential source rock, the rate of sediment accumulation of a formation as a function of time, and subsidence rate of a formation as a function of time. Subsidence rate includes total subsidence, compaction-induced subsidence, isostatic response to sediment load, and backstripped crustal subsidence (total subsidence minus the effects of compaction and isostatic response).

Interactive (SEDPAK) simulation of clastic and carbonate sediments in shelf to basin settings

Abstract SEDPAK is an interactive computer simulation which erects models of sedimentary geometries by infilling a two-dimensional basin from both sides with a combination of clastic sediment or in situ and transported carbonate sediments. The simulation program is implemented in “C” on an Apollo DN3000 workstation using graphical plotting functions. Data entry, including the initial basin configuration, local tectonic behavior, sealevel curves, amount and source direction of clastic sediment, and the growth rates of carbonates as a function of water depth is performed interactively. The modeled geometries of clastic and carbonate sediments evolve through time and respond to depositional processes that include tectonic movement, eustasy, and sedimentation. Clastic modeling includes sedimentary bypass, erosion, and sedimentation in alluvial and coastal plains, marine shelf, basin slope, and basin floor settings. Carbonate modeling includes progradation, the development of hard grounds, downslope aprons, keep up, catch up, back step, and drowned reef facies as well as lagoonal and epeiric facies. Also included in the model are extensional vertical faulting of the basin, sediment compaction, and isostatic response to sediment loading. Sediment geometries are plotted on a graphics terminal as they are computed, so the user can immediately view the results. Then, based on these observations, parameters can be changed repeatedly and the program rerun until the user is satisfied with the resultant geometry.

Thematic Set: Sequence stratigraphy: common ground after three decades of development

Sequence stratigraphy emphasizes changes in stratal stacking patterns in response to varying accommodation and sediment supply through time. Certain surfaces are designated as sequence or systems tract boundaries to facilitate the construction of realistic and meaningful palaeogeographic interpretations, which, in turn, allows for the prediction of facies and lithologies away from control points. Precisely which surfaces are selected as sequence boundaries varies from one sequence stratigraphic approach to another. In practice, the selection is often a function of which surfaces are best expressed, and mapped, within the context of each case study. This high degree of variability in the expression of sequence stratigraphic units and bounding surfaces requires the adoption of a methodology that is sufficiently flexible to accommodate the wide range of possible scenarios in the rock record. We advocate a model-independent methodology that requires the identification of all sequence stratigraphic units and bounding surfaces, which can be delineated on the basis of facies relationships and stratal stacking patterns using the available data. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture and predict the distribution of reservoir, source-rock, and seal facies.

Cretaceous chronostratigraphy, unconformities and eustatic sealevel changes in the sediments of Abu Dhabi, United Arab Emirates

Abstract The Cretaceous of the United Arab Emirates is divided into three major lithostratigraphic units separated by three regional unconformities: Lower Cretaceous Thamama Group (Berriasian-mid-Aptian); Mid-Cretaceous Wasia Group (Albian-Cenomanian, possibly Early Turonian) and Upper Cretaceous Aruma Group (Coniacian-Maastrichtian). In the United Arab Emirates, because of the abundance of subsurface data, it has been possible to relate the character of the Mesozoic shelf carbonates and their associated minor clastics and evaporites to eustatic sea-level changes. The patterns of sedimentation were driven by gentle tectonic subsidence punctuated by eustatic sea-level variations. Most of the carbonates are sheets formed in response to deposition during sea-level highstands while some of the upper Lower, Middle, and Upper Cretaceous carbonates contain build-ups that caught up with the sea-level highstands following rapid marine transgressions that initially stressed deposition. Shale-rich units deposited during sea-level lowstands and transgressive phases are common in the Cretaceous sequences. They occur in the Lower Cretaceous Lekhwair Formation and Bab Member. The Middle Cretaceous includes the lowstand Nahr Umr Formation and the basinal Shilaif, while the Upper Cretaceous contains the transgressive Laffan shales. The timing of the older unconformity at the end of the Lower Cretaceous deposition coincides with a pronounced eustatic lowering of sea-level during the middle Aptian. From the Valanginian through the middle Aptian the stratigraphy of the United Arab Emirates was a time of relative crustal stability, and reflects a steady rising of sea-level with minor fluctuations. The sea-level was relatively higher during the Middle Cretaceous. Tectonism in the form of the collision of the Omani plate with the Arabian plate, played an even greater role in controlling deposition during the Late Cretaceous but the unconformity at the end of the Late Cretaceous correlates well with a major sea-level low.

Geometry of carbonate bodies: A quantitative investigation of factors influencing their evolution

The geometries of the depositional surfaces of a series of carbonate bodies were modeled using a computer program that simulates the effects of physical and chemical environmental parameters on the deposition rates of carbonate accumulations. These parameters are: food supply (influence of nutrients), available light, temperature, salinity, dissolved oxygen, crustal movement and eustatic fluctuation.Geometries successfully modeled include: clinoformed margins with ramps and those with shelves, rimmed ramps with shelves, downslope buildups and buildups on shelves, all of which have analogs in the geologic record. Two basic geometric forms were produced: those resulting from a depth-dependent accumulation function with a single locus of maximum carbonate production extending outward from shore (Type I), and those with a laterally dependent function superimposed on a depth-dependent accumulation function (Type II). The second type has a locus of maximum carbonate production off shore but it may also be accompanied by a nearshore buildup similar to Type I. The geometries produced by the model approximate both modern and ancient examples.

Determination of paleoheat flux from vitrinite reflectance data, and from sterane and hopane isomer data

Abstract This paper describes paleoheat flux determinations by inversion from the independent thermal indicators vitrinite reflectance, and sterane and hopane isomer data, for a single well, well X, from the northern North Sea. When compared and contrasted these inversions give the same paleoheat flux variation with time to within error assessment of the data quality and the inversion schemes. This consistency provides firm corroborative support for the basic underpinning physics and chemistry used to describe the three indicators, since if a significant error were being made then no such consistency would be expected, contrary to determination. In addition, by exhaustive trial and error searches to determine a minimum residual mis-match between theory and observations, we obtain estimates of 50 ± 40 kJ mol −1 and 20 ± 40 kJ mol −1 for the activation energies of sterane isomerization and hopane isomerization, respectively. A neighboring well, Y, within 10 km of well X had sufficient hopane isomer data for inversion to be performed. It, too, yielded an activation energy of 30 ± 30 kJ mol −1 — confirming the result obtained in well X, as well as providing an assessment of the accuracy of this method of determining chemical parameters. Well Z, within 50 km, had just sufficient sterane isomer data for an inversion to be performed. It yielded an activation energy of 140 ± 100 kJ mol −1 — comparable to that from well X. The consistency of the three independently determined paleoheat flux variations with time, and of the chemical activation energies, strongly implies that the inversion of different downhole thermal indicator data is a valid technique, yielding consistent behavior. This is important to the assessment of the thermal history of source rocks since it can form the basis for a systematic method of assessing basinal thermal maturity, independent of model behaviors assumed for paleoheat flux without supporting evidence.

Estimating Unconformity Thicknesses from Vitrinite Reflectance Data: ABSTRACT

Combining variations of vitrinite reflectance with depth and burial history information enables estimation of the paleoheat flux-time derivative, s. In the presence of an unconformity, the residual r.m.s. fit of the paleoheat flux to the observed data is significantly improved when the unconformity thickness (h) is allowed to be a free variable. A best unconformity thickness that minimizes the residual r.m.s. fit to s can be determined, providing a new method of determining simultaneously paleoheat flux and unconformity thickness. End_of_Article - Last_Page 273------------

Graphical simulation of clastic margin progradation: Ulleung Basin, offshore Korea

The evolution of the southern part of the Ulleung Basin, East Sea (Sea of Japan) is analysed using a two-dimensional computer-based graphical simulation (SEDPAK) on a multichannel seismic profile. Iterative editing of the input data file includes various factors: initial basin configuration, local tectonic behavior, eustatic sea level, and the amounts and direction of clastic sedimentation. The sedimentary succession on the seismic profile can be divided into seven sequences bounded by sequence boundaries. The simulation relatively well reproduces evolving geometry shown on the interpreted seismic profile and burial history of sediments through time. The over 3000-m-thick sedimentary body is composed of sandstone and shale which have been accumulated in deltaic setting and as slumping or turbidites in a shelf-slope setting since the middle Miocene (16.5 Ma). The SEDPAK simulation shows all sequences except for the upper part of Sequence 5 which displays progradational clinoforms and onlapping transgressive units. It well reflects that the simulated sequences were formed during the mature stage of the back-arc basin development since the early Miocene. Scouring by incised valleys occurred extensively after the deposition of Sequence 5 on the margin, most obviously during the lowstand of sea level. Scouring may be related to the late Miocene thrusting and wrenching caused probably by collision of the Bonin Arc with the Amurian Plate. Simulation also shows the evolutionary development of the sedimentary sequences, which can track the burial history of individual layers throughout the run. When the 16.5 Ma surface is assumed to be the initial basin surface (the top of the pre-16.5 Ma sequence), the burial path of the 17.5 Ma to 16.5 Ma sequence reveals that it was at a depth of 3000 m at 12.5 Ma; 3200 to 3900 m at 6.3 Ma; at 3500 to 4200 m at 3.8 Ma; and at 4000 to 4700 m today. In addition, one can follow the burial history at a particular location using the time-depth-elevation plot for a specific column of the output within the simulated section. These features aid in the prediction of sedimentary facies distribution and geothermal history of the sequences.

Two dimensional restoration of seismic reflection profiles from Mozambique: technique for assessing rift extension histories

Seismic reflection data from offshore Mozambique between longitudes 25/sup 0/ and 26/sup 0/ and latitudes 34/sup 0/ and 35/sup 0/ reveals a V-shaped rift, the apex of which points northward, toward the coast. This study retraces the rifts extensional history by geometric reconstruction of seismic profiles, selected perpendicular to tectonic strike. Depth conversions are performed, followed by bed length and volume balancing to test the interpretations and calculate a total extension value for the extension factor. The sediments are then backstripped in sedimentary sequences, restoring the increments of throw on faults accordingly. After each sequence is removed, the sediments are decompacted in an attempt to recover the original volume prior to the sequence deposition. The extension factor is again calculated. This process is repeated down the sequences until the result is the pre-rift state of the basin. This analysis results in an extension estimate for each sequence-time increment, as a percentage of the total extension. From this method, a detailed crustal extension history is deduced, which, when coupled to the thermal history from subsidence backstripping and paleoheatflow studies, could be used in the basin analysis assessment of the oil potential of this and other rifts.